1. Field of the Invention
The present invention relates generally to the field of optical communication and, more specifically, to an improved optic rotary communication system that provides optical communication offset from the rotary axis on applications such as spacecrafts and gimbals.
2. Description of Related Art
There has been an increase in the past years of optical communication systems such as the use of fiber optic signal transmission. One problem confronted over the years has been the effective transmission of optical signals onto or off of relatively rotating devices in what has been referred to as optical rotary joints. This problem exists for example in the contexts of spacecraft, gimbals, or missile heads where one portion rotates relative to the other portion. In those situations, like others, the design of communication apparatus commonly includes a rotating device that lies co-axial with a non-rotating device and an axial portion intermediate the rotating and non-rotating devices.
Some solutions to the problem have included converting the optical signal to an electrical one at the input to a rotary brush-and-slip ring interconnector and then converting the signal back to an optical signal at the output of the interconnector. This, however, introduces problems of wear and contact integrity with corresponding signal degradation, limited bandwidth, loss of signal, and increased noise by changing from optical to electrical and vice versa. Beam waveguides have also been utilized but have resulted in problems of mirror misalignment and bulky sized components which are directly related to the size of the wavelengths.
One design solution has suggested the use of co-axial optical cables like that described in Marrone, Winch Mounted Optical Data Transmission Cable With Fluid Coupling, U.S. Pat. No. 3,922,063, wherein interfacing axially aligned optical windows are connected to the ends of optical cables. Positioned between the optical windows is an optically transparent fluid having the same optical index of refraction as the fiber optic transmission path to thereby affect an optical coupling between the two cables. Another example of co-axial coupling is taught by Henderson et al. Rotary Fiber Optical Wave Guide Coupling, U.S. Pat. No. 4,124,272. A pair of axially aligned ferrules hold two ends of fiber optic cables therein. The ferrules are held in an axial position by abutment at their inner ends in a pair of alignment caps mounted in two relatively rotatable housing members. A pair of retaining sleeves screwed into the housing members cooperate with the ferrules to obtain radial and axial positioning.
However, co-axial cable designs have problems. The movement of the rotating member is often limited. The required components are often heavy in weight and large in size. One major problem encountered in the co-axial cable design has been the misalignment of the ends of the optical cables which causes signal attenuation. The associated problems of lateral misalignment, end separation, and angular separation in the context of co-axial cables is further described in Dorsey, Fiber Optical Rotary Joints--A Review, I.E.E.E. Transactions On Components, Hybrids, and Manufacturing Technology, Vol. CHMT-5, No. 1, March 1982.
Other optical rotary joint designs have included apparatus that transmits the optical signal outside of the rotary axis. For example, Fitch, Rotary Optical Coupler, U.S. Pat. No. 4,165,913 describes an apparatus that does not transmit the signal between the interfacing surfaces of the rotating and non-rotating members. Instead, an optical fiber is wrapped around the exterior of a surface of the rotating member that does not interface with the non- rotating member. The surface of the optical fiber is roughened to permit the optical signal to be transmitted laterally through the fiber wall. A stationary light detector positioned outside of the rotating member and adjacent, the optical fiber receives the signal being transmitted through the fiber wall as the rotating member rotates. A major problem with this technique is that scattering light out the sides of the fiber is very inefficient.
Another optical rotary joint that provides for optical transmission outside of the rotary axis is shown in Iverson, Optical Sliprings, U.S. Pat. No. 4,027,945. A signal is transmitted into the rotating member through fiber bundles which terminates at the surface interfacing the non-rotating member in concentric, annular bundle ends. At the interfacing surface of the non-rotating member, the signal is transmitted from the rotating member to axially aligned, corresponding concentric annular bundle ends. Opaque bundle coverings or cladding within the rotating and non-rotating members serve to isolate the signals in each channel and maintain bundle shape. It is evident, however, that alignment problems can still present a significant problem in the bundle design, as in the above co-axial cable designs.
As can be appreciated the problems of a hostile environment and weight limitations in a space application provide even greater problems. Accordingly, there is still a requirement in the prior art to provide a relatively light weight optical communication joint for application in outer space and other environments.